Most speech coding systems in use today are based on telephone-bandwidth narrowband speech. The telephone-bandwidth is nominal limited to about 300-3400 Hz and sampled at a rate of 8 kHz. This limitation built into the conventional telephone system dates back to when the service was established. The inherent bandwidth limitations in the conventional public switched telephone network (PSTN) impose a limit on communication quality.
The increasing penetration of the end-to-end digital networks, such as 2G and 3G wireless systems, will permit the use of wider speech bandwidth that will offer communication quality that significantly surpasses that of the PSTN and creates the sensation of face-to-face communication.
In wideband speech coding (AMR-WB), the signal is for example sampled at 16 kHz, and the band is for example limited to 100-7000 Hz, which results in speech quality close to that of face-to-face communication.
Already today's networks support the Out of Band Transcoder Control (OoBTC) function for codec negotiation.
However, the negotiation so far supports only narrowband codec types and PCM.
OoBTC function provides an algorithm optimized to negotiate the best speech codec at call setup, providing improved Core Network bandwidth usage and improved speech quality.
International Patent application PCT/EP2004/002229 addresses the problem of how to provide optimal data quality and/or to minimize the bandwidth needs in the telecommunications core network and suggests an improved codec list handling for GSM/WCDMA networks.
There, the codec list was divided into a two part list, whereby one part comprises so-called “direct codecs”, which are codecs that are supported by the terminal, the radio access network and the Media Gateway, where the radio access network is connected to, and the other part so-called “transcoding codecs” or “indirect codecs”, which are supported by the Media Gateway, but not by the terminal or not by the RAN, and therefore afford transcoding in the Media Gateway. The list could be separated by a separator such as a common or default codec.
The invention adds to the improved codec list structure how a specific handling methods of a wideband codec could be implemented as well as corresponding handling logic.
Furthermore, as already stated, wideband codecs may offer superior speech quality when used end-to-end.
However, it has to be noted, that in order to achieve the improved speech quality, the wideband encoding algorithm typically requires in the order of 2-4 times more processing power of an encoding/decoding DSP than would be necessary for a narrowband codec.
Hence, in terms of processing power, a wideband codec is expensive.
However, the speech quality enhancements related to such a wideband codec are achieved only if the wideband codec is used end-to-end. Any narrowband link within the transmission path will lead to only a narrowband quality transported end-to-end.
It has also to be noted, that the call set-up may change during a call leading to a situation that the wideband codec would become available or would no longer be available.